Wafer probe station having a skirting component

ABSTRACT

A probe station includes a fully guarded chuck assembly and connector mechanism for increasing sensitivity to low-level currents while reducing settling times. The chuck assembly includes a wafer-supporting first chuck element surrounded by a second chuck element having a lower component, skirting component and upper component each with a surface portion extending opposite the first element for guarding thereof. The connector mechanism is so connected to the second chuck element as to enable, during low-level current measurements, the potential on each component to follow that on the first chuck element as measured relative to an outer shielding enclosure surrounding each element. Leakage current from the first chuck element is thus reduced to virtually zero, hence enabling increased current sensitivity, and the reduced capacitance thus provided by the second chuck element decreases charging periods, hence reducing settling times. With similar operation and effect, where any signal line element of the connector mechanism is arranged exterior of its corresponding guard line element, such as adjacent the chuck assembly or on the probe-holding assembly, a guard enclosure is provided to surround and fully guard such signal line element in interposed relationship between that element and the outer shielding enclosure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/678,549, filed Oct. 2, 2003, now U.S. Pat. No. 6,980,012; which is acontinuation of U.S. patent application Ser. No. 10/274,068, filed Oct.17, 2002, now U.S. Pat. No. 6,720,782; which is a continuation of U.S.patent application Ser. No. 10/003,948, filed Oct. 30, 2001, now U.S.Pat. No. 6,492,822; which is a continuation of U.S. patent applicationSer. No. 09/784,231, filed Feb. 13, 2001, now U.S. Pat. No. 6,335,628;which is a continuation of U.S. patent application Ser. No. 08/855,735,filed May 9, 1997, now U.S. Pat. No. 6,232,788; which is a continuationof U.S. patent application Ser. No. 08/508,325 filed Jul. 27, 1995, nowU.S. Pat. No. 5,663,653; which is a continuation of U.S. patentapplication Ser. No. 08/100,494, filed Aug. 2, 1993, now U.S. Pat. No.5,457,398; which is a continuation-in-part of U.S. patent applicationSer. No. 07/896,853 filed Jun. 11, 1992, now U.S. Pat. No. 5,345,170;and

this application is also a continuation of U.S. patent application Ser.No. 10/678,549, filed Oct. 2, 2003, now U.S. Pat. No. 6,980,012; whichis a continuation of U.S. patent application Ser. No. 10/274,068, filedOct. 17, 2002, now U.S. Pat. No. 6,720,782; which is a continuation ofU.S. patent application Ser. No. 10/003,948, filed Oct. 30, 2001, nowU.S. Pat. No. 6,492,822; which is a continuation of U.S. patentapplication Ser. No. 09/784,231, filed Feb. 13, 2001, now U.S. Pat. No.6,335,628; which is a continuation of U.S. patent application Ser. No.08/855,735, filed May 9, 1997, now U.S. Pat. No. 6,232,788; which is acontinuation of U.S. patent application Ser. No. 08/508,325 filed Jul.27, 1995, now U.S. Pat. No. 5,663,653; which is a continuation-in-partof U.S. patent application Ser. No. 08/417,982 filed Apr. 6, 1995, nowU.S. Pat. No. 5,532,609; which is a division of U.S. patent applicationSer. No. 08/245,581 filed May 18, 1994, now U.S. Pat. No. 5,434,512;which is a division of U.S. patent application Ser. No. 07/896,853 filedJun. 11, 1992, now U.S. Pat. No. 5.345.170.

BACKGROUND OF THE INVENTION

The present invention is directed to probe stations adapted for makinghighly accurate low-current and low-voltage measurements of wafers andother electronic test devices. More particularly, the invention relatesto such a probe station having a guarding system for preventing currentleakage, a Kelvin connection system to eliminate voltage losses causedby line resistances, and an electromagnetic interference (EMI) shieldingsystem.

The technique of guarding to minimize current leakage during low-currentmeasurements, the use of Kelvin connections for low-voltagemeasurements, and the provision of EMI shielding are all well known anddiscussed extensively in the technical literature. See, for example, anarticle by William Knauer entitled “Fixturing forLow-Current/Low-Voltage Parametric Testing,” appearing in EvaluationEngineering, November, 1990, pages 150-153. See also Hewlett-Packard,“Application Note 356-HP 4142B Modular DC Source/Monitor PracticalApplication,” (1987) pages 1-4, and Hewlett-Packard, H-P Model 4284APrecision LCR Meter, Operation Manual (1991) pages 2-1, 6-9, and 6-15.

In guarding applications, a conductor surrounding or otherwise closelyadjacent to a low-current line or circuit is maintained at the samepotential as the line or circuit to reduce leakage currents therefrom,so that low-current measurements can be made accurately.

Kelvin connections compensate for voltage losses caused by lineresistances which-would otherwise cause errors in low-voltagemeasurements. This is accomplished by providing a source line and ameasurement line (also referred to commonly as “force” and “sense”lines, respectively) to an interconnection point (the Kelvin connection)which is as close to the test device as possible. A high-impedancevoltmeter is connected to this interconnection point through themeasurement line to accurately detect the voltage without anysignificant flow of current or resultant voltage drop in the measurementline. This avoids the error which would otherwise occur if the voltmeterwere to detect the voltage through the source line, due to the voltagedrop that occurs in the source line.

Probe stations have previously been used for conducting tests withguarding, Kelvin connection, and EMI shielding techniques. However thecustom set-up of such probe stations required for guarding and Kelvinconnection procedures is time-consuming and, in some instances, limitedas to effectiveness. For example, in an article by Yousuke Yamamoto,entitled “A Compact Self-Shielding Prober for Accurate Measurement ofOn-Wafer Electron Devices,” appearing in IEEE Transactions onInstrumentation and Measurement, Volume 38, No. 6, December, 1989, pages1088-1093, a probe station is shown having a respective detachabletriaxial connector mounted on the probe card and the chuck assemblywhich supports the test device. The intermediate connector element of atriaxial connector normally is utilized for guarding. purposes. Howeverthe chuck assembly shown has only a chuck and a shield, with no separateguarding structure to which the intermediate connector element could beconnected. Accordingly significant time-consuming alteration of such astation would be required to obtain both a guarded and shielded chuckassembly. The probes on the probe card, on the other hand, are bothguarded and shielded; however there is no means of enabling each probeto be moved independently of the others in unison with its guard andshield to accommodate different contact patterns of test devices, thussacrificing flexibility of the probe station. Also, there is noprovision for Kelvin connections on the chuck assembly, which wouldrequire more than a single triaxial connector as shown.

Chuck assemblies are available which provide guarding and shieldingcomponents. For example, Temptronic Corporation of Newton, Mass. marketsa thermal chuck assembly atop which is mounted an “add-on” supportingsurface for the test device, with a copper guarding layer interposedbetween the add-on surface and the underlying chuck assembly andinsulated from each by respective sheets of insulating material. Thisstructure permits a signal line to be soldered to the add-on surface, aguard line to be soldered to the copper guarding layer, and a groundline to be soldered to the underlying chuck assembly which can thenserve as a shield. However such wiring requires time-consuming set-up,particularly if Kelvin connections are also required. Moreover, the useof sheet insulation to insulate the copper guarding layer, from theadd-on surface and the underlying chuck assembly fails to provide as lowa dielectric constant between the respective elements as is desirable tominimize leakage currents in view of the low level of current to bemeasured.

With respect to -probe stations that are designed to accommodate themeasurement of low levels of current, a sensitivity threshold isnormally encountered below which further improvements in currentsensitivity are difficult to reliably achieve. In most commercial probestations that are of such design, this sensitivity threshold istypically reached at about 20-50 femtoamps. However, improvements indevice fabrication and in the capabilities of commercially availabletest instrumentation make it desirable to reduce the sensitivitythreshold to a level reliably within the single digit femtoamp range.

A particular difficulty encountered in low-level current measurements isthe excessive time required for measurement voltages to stabilize withreference to the device under test after a shift in voltage has occurredat the electrical input to the probe station. This problem of excessivesettling time, as it is referred to, increases as the level of currentunder measurement is reduced. That is, due to the residual capacitanceexisting between spaced apart conductors in the region surrounding theimmediate test site, a certain amount of time is required for theconductors that are in direct connection with the test device to fullycharge or discharge to their desired voltages, and the time requiredwill increase as the current through the device decreases. If theresidual capacitance and the degree of input voltage shift aremoderately large and if the level of current being measured ismoderately small, the probe station operator can encounter settlingtimes that are upwards of two or three minutes. Clearly, then, it isdesirable that settling times be generally reduced in order to reduceoverall measurement time, particularly where the device under test is awafer containing large numbers of discrete devices, each of which mayindividually require low-level current testing.

In addition to settling effects, measurements of low level currents arealso susceptible to error due to electrical discharge effects whichoccur because of the acceptance and release of charge by nonconductorsin the region surrounding the immediate test site. At very low currents,these discharge effects can significantly-distort measurement values andhence contribute to unacceptable levels of measurement instability.

SUMMARY OF THE INVENTION

The present invention solves the foregoing drawbacks of the prior probestations by providing a probe station having integrated and ready-to-useguarding, Kelvin connection and shielding systems, both for individuallymovable probes and for the chuck assembly.

In further preferred embodiments of the invention, an improved guardingsystem is provided for accurate and rapid measurement of very low-levelcurrents.

The chuck assembly of the present invention may in preferred embodimentsthereof comprise at least first, second and third chuck assemblyelements electrically insulated from one another and positioned atprogressively greater distances from the probe(s) along the axis ofapproach between them. At least one detachable electrical connectorassembly is provided on the chuck assembly having respective connectorelements connected matingly to the first and second chuck assemblyelements, respectively, so as to provide a ready-to-use guarding systemrequiring only the easy detachable connection of a guarded cable to theconnector assembly for immediate operability.

Preferably, a second such detachable electrical connector assembly isalso provided having its corresponding connector elements connected, inparallel with those of the first connector assembly, to the first andsecond chuck assembly elements so as to provide a ready-to-use guardedKelvin connection on the chuck assembly which becomes immediatelyoperable by the easy detachable connection of a second guarded cable tothe second connector assembly. Thus one cable serves as a guarded sourceline and the other serves as a guarded measurement line.

Leakage currents in the chuck assembly are preferably minimized by thefact that the three chuck assembly elements are electrically insulatedfrom one another by distributed patterns of dielectric spacers, ratherthan continuous dielectric sheets, so that large air gaps are providedbetween the respective chuck assembly elements to reduce the dielectricconstant in the gaps between the elements.

In further preferred embodiments of the present invention, the secondchuck assembly element is provided with respective upper, lower andskirting components to provide full guarding for the first chuckassembly element. In particular, respective surface portions on theupper, lower and skirting components extend opposite the upper, lowerand peripheral surfaces, respectively, of the first chuck assemblyelement. Furthermore, a connector mechanism is provided that enables anonzero potential to be established on the first chuck assembly elementrelative to ground, that is, relative to the outer shielding enclosure,and a substantially equal potential to be established on the secondchuck assembly element.

In accordance, then, with a preferred method of use, the exemplary chuckassembly structure just described is energized via the connectormechanism so that the potential on the first element is effectivelymatched by a substantially equal potential on the second element wherebyvirtually no potential difference is developed in the region between theelements. As a result of this relationship and the arrangement ofcomponents of the second chuck assembly element, leakage current fromthe first chuck assembly element is reduced to virtually zero whichenables low-level currents to be measured with increased sensitivity.Furthermore, with respect to low-level current measurements, settlingtimes during startup and switchover phases of operation are reduced.That is, the second chuck assembly element, unlike the first, acquiresor releases charge at a rate not limited by the large effectiveresistance presented by the device under test. Accordingly, therespective guarding components are able to achieve their full potentialrelatively quickly even though they are directly coupled capacitively toconductive surfaces of large area such as those on the outer shieldingenclosure. The respective guarding components also serve as an effectivebarrier to stray radiation to the extent they are interposed between theelement emitting such radiation and the first chuck assembly element.Therefore, relative even to the low levels of current being measured,the potential error or instability in each measurement is reduced to aninsignificant level.

Individually movable probe holders are provided having not onlyready-to-use guarded signal line cables and Kelvin connection cables,but also respective shields for the cables of each probe, the shieldsbeing movable independently in unison with each probe separately.

Where a line element of the connector mechanism that carries the signalis arranged exterior of its corresponding guard element, such as whereit is separated out from the guard element for interconnection withanother signal element, preferably a conductive guard enclosure isprovided which surrounds the signal line element in interposedrelationship between such element and the outer shielding enclosure.Furthermore, when a nonzero potential is established during low-levelcurrent measurement on the signal line element relative to ground, thatis, relative to the outer shielding enclosure, preferably the connectormechanism is so connected to the guard enclosure as to enable asubstantially equal potential to be established on the guard enclosure.

The signal line guarding system just described can thus be energized viathe connector mechanism so that virtually no potential difference isdeveloped between the signal line element and its surrounding guardenclosure. Hence, the level of leakage current flowing away from thesignal line element is reduced to virtually zero which enables low-levelcurrents in the system to be measured with increased sensitivity. Also,since there is a reduction in the combined area of the conductivesurfaces to which the signal line element is capacitively coupled, lessenergy transfer and time is required for this line element to acquireits full potential, so that settling-time is reduced. Moreover, if anytransient shifts in electrical state should occur in relation to anynonconductor or conductor located outside the guard enclosure, this willhave virtually no effect on the signal line element due to the effectivebarrier against radiation provided by the conductive guard enclosure, sothat measurement instability is reduced.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial front view of an exemplary embodiment of a waferprobe station constructed in accordance with the present invention.

FIG. 2 is a top view of the wafer probe station of FIG. 1.

FIG. 2A is a partial top view of the wafer probe station of FIG. 1 withthe enclosure door shown partially open.

FIG. 3 is a partially sectional and partially schematic front view ofthe probe station of FIG. 1.

FIG. 3A is an enlarged sectional view taken along line 3A-3A of FIG. 3.

FIG. 4 is a top view of the sealing assembly where the motorizedpositioning mechanism extends through the bottom of the enclosure.

FIG. 5A is an enlarged top detail view taken along line 5A-5A of FIG. 1.

FIG. 5B is an enlarged top sectional view taken along line 5B-5B of FIG.1.

FIG. 6 is a partially schematic top detail view of the chuck assembly,taken along line 6-6 of FIG. 3.

FIG. 7 is a partially sectional front view of the chuck assembly of FIG.6.

FIG. 8 is a partially sectional side view of a probe holder and probe.

FIG. 9 is a partially sectional bottom view taken along line 9-9 of FIG.8.

FIG. 10 is a partially sectional front view of an alternative exemplaryembodiment of a wafer probe station constructed in accordance with thepresent invention.

FIG. 11 is a front detail view showing the lower elements of the chuckassembly of the wafer probe station of FIG. 10 with hidden portionsshown in cut-away view.

FIG. 12 is a partial top detail view showing the connector mechanism andthe lower elements of the chuck assembly as viewed along line 12-12 ofFIG. 10.

FIG. 13 is a partial top view of the wafer probe station of FIG. 10 withthe outer enclosure door shown partially open.

FIG. 14 is a bottom view of an optional conductive panel in position onthe upper guard component as viewed along line 14-14 in FIG. 10.

FIG. 15 is a partially sectional side view of an alternative exemplaryprobe holder which is suitable for use in association with the waferprobe station of FIG. 10.

FIG. 16 is a partially sectional bottom view taken along line 16-16 ofFIG. 15 with hidden portions shown in cut-away view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS General Arrangement of ProbeStation

With reference to FIGS. 1, 2 and 3, an exemplary embodiment of the probestation of the present invention comprises a base 10 (shown partially)which supports a platen 12 through a number of jacks 14 a, 14 b, 14 c,14 d which selectively raise and lower the platen vertically relative tothe base by a small increment (approximately one-tenth of an inch) forpurposes to be described hereafter. Also supported by the base 10 of theprobe station is a motorized positioner 16 having a rectangular plunger18 which supports a movable chuck assembly 20 for supporting a wafer orother test device. The chuck assembly 20 passes freely through a largeaperture 22 in the platen 12 which permits the chuck assembly to bemoved independently of the platen by the positioner 16 along X, Y and Zaxes, i.e. horizontally along two mutually-perpendicular axes X and Y,and vertically along the Z axis. Likewise, the platen 12, when movedvertically by the jacks 14, moves independently of the chuck assembly 20and the positioner 16.

Mounted atop the platen 12 are multiple individual probe positionerssuch as 24 (only one of which is shown), each having an extending member26 to which is mounted a probe holder 28 which in turn supports arespective probe 30 for contacting wafers and other test devices mountedatop the chuck assembly 20. The probe positioner 24 has micrometeradjustments 34, 36 and 38 for adjusting the position of the probe holder28, and thus the probe 30, along the X, Y and Z axes respectively,relative to the chuck assembly 20. The Z axis is exemplary of what isreferred to herein loosely as the “axis of approach” between the probeholder 28 and the chuck assembly 20, although directions of approachwhich are neither vertical nor linear, along which the probe tip andwafer or other test device are brought into contact with each other, arealso intended to be included within the meaning of the term “axis ofapproach.” A further micrometer adjustment 40 adjustably tilts the probeholder 28 to adjust planarity of the probe with respect to the wafer orother test device supported by the chuck assembly 20. As many as twelveindividual probe positioners 24, each supporting a respective probe, maybe arranged on the platen 12 around the chuck assembly 20 so as toconverge radially toward the chuck assembly similarly to the spokes of awheel. With such an arrangement, each individual positioner 24 canindependently adjust its respective probe in the X, Y and Z directions,while the jacks 14 can be actuated to raise or lower the platen 12 andthus all of the positioners 24 and their respective probes in unison.

An environment control outer enclosure is composed of an upper boxportion 42 rigidly attached to the platen 12, and a lower box portion 44rigidly attached to the base 10. Both portions are made of steel orother suitable electrically conductive material to provide EMIshielding. To accommodate the small vertical movement between the twobox portions 42 and 44 when the jacks 14 are actuated to raise or lowerthe platen 12, an electrically conductive resilient foam gasket 46,preferably composed of silver or carbon-impregnated silicone, isinterposed peripherally at their mating juncture at the front of theenclosure and between the lower portion 44 and the platen 12 so that anEMI, substantially hermetic, and light seal are all maintained despiterelative vertical movement between the two box portions 42 and 44. Eventhough the upper box portion 42 is rigidly attached to the platen 12, asimilar gasket 47 is preferably interposed between the portion 42 andthe top of the platen to maximize sealing.

With reference to FIGS. 5A and 5B, the top of the upper box portion 42comprises an octagonal steel box 48 having eight side panels such as 49a and 49 b through which the extending members 26 of the respectiveprobe positioners 24 can penetrate movably. Each panel comprises ahollow housing in which a respective sheet 50 of resilient foam, whichmay be similar to the above-identified gasket material, is placed. Slitssuch as 52 are partially cut vertically in the foam in alignment withslots 54 formed in the inner and outer surfaces of each panel housing,through which a respective extending member 26 of a respective probepositioner 24 can pass movably. The slitted foam permits X, Y and Zmovement of the extending members 26 of each probe positioner, whilemaintaining the EMI, substantially hermetic, and light seal provided bythe enclosure. In four of the panels, to enable a greater range of X andY movement, the foam sheet 50 is sandwiched between a pair of steelplates 55 having slots 54 therein, such plates being slidabletransversely within the panel housing through a range of movementencompassed by larger slots 56 in the inner and outer surfaces of thepanel housing.

Atop the octagonal box 48, a circular viewing aperture 58 is provided,having a recessed circular transparent sealing window 60 therein. Abracket 62 holds an apertured sliding shutter 64 to selectively permitor prevent the passage of light through the window. A stereoscope (notshown) connected to a CRT monitor can be placed above the window toprovide a magnified display of the wafer or other test device and theprobe tip for proper probe placement during set-up or operation.Alternatively, the window 60 can be removed and a microscope lens (notshown) surrounded by a foam gasket can be inserted through the viewingaperture 58 with the foam providing EMI, hermetic and light sealing.

The upper box portion 42 of the environment control enclosure alsoincludes a hinged steel door 68 which pivots outwardly about the pivotaxis of a hinge 70 as shown in FIG. 2A. The hinge biases the doordownwardly toward the top of the upper box portion 42 so that it forms atight, overlapping, sliding peripheral seal 68 a with the top of theupper box portion. When the door is open, and the chuck assembly 20 ismoved by the positioner 16 beneath the door opening as shown in FIG. 2A,the chuck assembly is accessible for loading and unloading.

With reference to FIGS. 3 and 4, the sealing integrity of the enclosureis likewise maintained throughout positioning movements by the motorizedpositioner 16 due to the provision of a series of four sealing plates72, 74, 76 and 78 stacked slidably atop one another. The sizes of theplates progress increasingly from the top to the bottom one, as do therespective sizes of the central apertures 72 a, 74 a, 76 a and 78 aformed in the respective plates 72, 74, 76 and 78, and the aperture 79 aformed in the bottom 44 a of the lower box portion 44. The centralaperture 72 a in the top plate 72 mates closely around the bearinghousing 18 a of the vertically-movable plunger 18. The next plate in thedownward progression, plate 74, has an upwardly-projecting peripheralmargin 74 b which limits the extent to which the plate 72 can slideacross the top of the plate 74. The central aperture 74 a in the plate74 is of a size to permit the positioner 16 to move the plunger 18 andits bearing housing 18 a transversely along the X and Y axes until theedge of the top plate 72 abuts against the margin 74 b of the plate 74.The size of the aperture 74 a is, however, too small to be uncovered bythe top plate 72 when such abutment occurs, and therefore a seal ismaintained between the plates 72 and 74 regardless of the movement ofthe plunger 18 and its bearing housing along the X and Y axes. Furthermovement of the plunger 18 and bearing housing in the direction ofabutment of the plate 72 with the margin 74 b results in the sliding ofthe plate 74 toward the peripheral margin 76 b of the next underlyingplate 76. Again, the central aperture 76 a in the plate 76 is largeenough to permit abutment of the plate 74 with the margin 76 b, butsmall enough to prevent the plate 74 from uncovering the aperture 76 a,thereby likewise maintaining the seal between the plates 74 and 76.Still further movement of the plunger 18 and bearing housing in the samedirection causes similar sliding of the plates 76 and 78 relative totheir underlying plates into abutment with the margin 78 b and the sideof the box portion 44, respectively, without the apertures 78 a and 79 abecoming uncovered. This combination of sliding plates and centralapertures of progressively increasing size permits a full range ofmovement of the plunger 18 along the X and Y axes by the positioner 16,while maintaining the enclosure in a sealed condition despite suchpositioning movement. The EMI sealing provided by this structure iseffective even with respect to the electric motors of the positioner 16,since they are located below the sliding plates.

Chuck Assembly

With particular reference to FIGS. 3, 6 and 7, the chuck assembly 20 isof a unique modular construction usable either with or without anenvironment control enclosure. The plunger 18 supports an adjustmentplate 79 which in turn supports first, second and third chuck assemblyelements 80, 81 and 83, respectively, positioned at progressivelygreater distances from the probe(s) along the axis of approach. Thelower chuck assembly element 83 is a conductive rectangular stage orshield 83 which detachably mounts conductive elements 80 and 81 ofcircular shape. In addition to having a lower surface 160 and aperipheral surface 162, the upper chuck assembly element 80 has a planarupwardly-facing wafer-supporting or upper surface 82 having an array ofvertical apertures 84 therein. These apertures communicate withrespective chambers separated by O-rings 88, the chambers in turn beingconnected separately to different vacuum lines 90 a, 90 b, 90 c (FIG. 6)communicating through separately-controlled vacuum valves (not shown)with a source of vacuum. The respective vacuum lines selectively connectthe respective chambers and their apertures to the source of vacuum tohold the wafer, or alternatively isolate the apertures from the sourceof vacuum to release the wafer, in a conventional manner. The separateoperability of the respective chambers and their corresponding aperturesenables the chuck to hold wafers of different diameters.

In addition to the circular elements 80 and 81, auxiliary chucks such as92 and 94 are detachably mounted on the corners of the element 83 byscrews (not shown) independently of the elements 80 and 81 for thepurpose of supporting contact substrates and calibration substrateswhile a wafer or other test device is simultaneously supported by theelement 80. Each auxiliary chuck 92, 94 has its own separateupwardly-facing planar surface 100, 102 respectively, in parallelrelationship to the surface 82 of the element 80. Vacuum apertures 104protrude through the surfaces 100 and 102 from communication withrespective chambers within the body of each auxiliary chuck. Each ofthese chambers in turn communicates through a separate vacuum line and aseparate independently-actuated vacuum valve (not shown) with a sourceof vacuum, each such valve selectively connecting or isolating therespective sets of apertures 104 with respect to the source of vacuumindependently of the operation of the apertures 84 of the element 80, soas to selectively hold or release a contact substrate or calibrationsubstrate located on the respective surfaces 100 and 102 independentlyof the wafer or other test device. An optional metal shield 106 mayprotrude upwardly from the edges of the element 83 to surround or skirtthe other elements 80, 81 and the auxiliary chucks 92, 94.

All of the chuck assembly elements 80, 81 and 83, as well as theadditional chuck assembly element 79, are electrically insulated fromone another even though they are constructed of electrically conductivemetal and interconnected detachably by metallic screws such as 96. Withreference to FIGS. 3 and 3A, the electrical insulation results from thefact that, in addition to the resilient dielectric O-rings 88,dielectric spacers 85 and dielectric washers 86 are provided. These,coupled with the fact that the screws 96 pass through oversizedapertures in the lower one of the two elements which each screw joinstogether thereby preventing electrical contact between the shank of thescrew and the lower element, provide the desired insulation. As isapparent in FIG. 3, the dielectric spacers 85 extend over only minorportions of the opposing surface areas of the interconnected chuckassembly elements, thereby leaving air gaps between the opposingsurfaces over major portions of their respective areas. Such air gapsminimize the dielectric constant in the spaces between the respectivechuck assembly elements, thereby correspondingly minimizing thecapacitance between them and the ability for electrical current to leakfrom one element to another. Preferably the spacers and washers 85 and86, respectively, are constructed of a material having the lowestpossible dielectric constant consistent with high dimensional stabilityand high volume resistivity. A suitable material for the spacers andwashers is glass epoxy, or acetal homopolymer marketed under thetrademark Delrin by E.I. DuPont.

With reference to FIGS. 6 and 7, the chuck assembly 20 also includes apair of detachable electrical connector assemblies designated generallyas 108 and 110, each having at least two conductive connector elements108 a, 108 b and 110 a, 110 b, respectively, electrically insulated fromeach other, with the connector elements 108 b and 110 b preferablycoaxially surrounding the connector elements 108 a and 110 a as guardstherefor. If desired, the connector assemblies 108 and 110 can betriaxial in configuration so as to include respective outer shields 108c, 110 c surrounding the respective connector elements 108 b and 110 b,as shown in FIG. 7. The outer shields 108 c and 110 c may, if desired,be connected electrically through a shielding box 112 and a connectorsupporting bracket 113 to the chuck assembly element 83, although suchelectrical connection is optional particularly in view of thesurrounding EMI shielding enclosure 42, 44. In any case, the respectiveconnector elements 108 a and 110 a are electrically connected inparallel to a connector plate 114 matingly and detachably connectedalong a curved contact surface 114 a by screws 114 b and 114 c to thecurved edge of the chuck assembly element 80. Conversely, the connectorelements 108 b and 110 b are connected in parallel to a connector plate116 similarly matingly connected detachably to element 81. The connectorelements pass freely through a rectangular opening 112 a in the box 112,being electrically insulated from the box 112 and therefore from theelement 83, as well as being electrically insulated from each other. Setscrews such as 118 detachably fasten the connector elements to therespective connector plates 114 and 116.

Either coaxial or, as shown, triaxial cables 118 and 120 form portionsof the respective detachable electrical connector assemblies 108 and110, as do their respective triaxial detachable connectors 122 and 124which penetrate a wall of the lower portion 44 of the environmentcontrol enclosure so that the outer shields of the triaxial connectors122, 124 are electrically connected to the enclosure. Further triaxialcables 122 a, 124 a are detachably connectable to the connectors 122 and124 from suitable test equipment such as a Hewlett-Packard 4142B modularDC source/monitor or a Hewlett-Packard 4284A precision LCR meter,depending upon the test application. If the cables 118 and 120 aremerely coaxial cables or other types of cables having only twoconductors, one conductor interconnects the inner (signal) connectorelement of a respective connector 122 or 124 with a respective connectorelement 108 a or 110 a, while the other conductor connects theintermediate (guard) connector element of a respective connector 122 or124 with a respective connector element 108b, 110 b.

In any case, the detachable connector assemblies 108, 110, due to theirinterconnections with the two connector plates 114, 116, provideimmediately ready-to-use signal and guard connections to the chuckassembly elements 80 and 81, respectively, as well as ready-to-useguarded Kelvin connections thereto. For applications requiring onlyguarding of the chuck assembly, as for example the measurement oflow-current leakage from a test device through the element 80, it isnecessary only that the operator connect a single guarded cable 122 afrom a test instrument such as a Hewlett-Packard 4142B modular DCsource/monitor to the detachable connector 122 so that a signal line isprovided to the chuck assembly element 80 through the connector element108 a and connector plate 114, and a guard line is provided to theelement 81 through the connector element 108 b and connector plate 116.Alternatively, if a Kelvin connection to the chuck assembly is desiredfor low-voltage measurements, such as those needed for measurements oflow capacitance, the operator need merely attach a pair of cables 122 aand 124 a to the respective connectors 122, 124 from a suitable testinstrument such as a Hewlett-Packard 4284A precision LCR meter, therebyproviding both source and measurement lines to the element 80 throughthe connector elements 108 a and 110 a and connector plate 114, andguarding lines to the element 81 through the connector elements 108 band 110 b and connector plate 116.

Probe Assembly

With reference to FIGS. 5B, 8 and 9, respective individually movableprobes 30 comprising pairs of probe elements 30a are supported byrespective probe holders 28 which in turn are supported by respectiveextending portions 26 of different probe positioners such as 24. Atopeach probe positioner 24 is a shield box 126 having a pair of triaxialconnectors 128, 130 mounted thereon with respective triaxial cables 132entering each triaxial connector from a suitable test instrument asmentioned previously. Each triaxial connector includes a respectiveinner connector element 128 a, 130 a, an intermediate connector element128 b, 130 b, and an outer connector element 128 c, 130 c in concentricarrangement. Each outer connector element 128 c, 130 c terminates byconnection with the shield box 126. Conversely, the inner connectorelements 128 a, 130 a, and the intermediate connector elements 128 b,130 b, are connected respectively to the inner and outer conductors of apair of coaxial cables 134, 136 which therefore are guarded cables. Eachcable 134, 136 terminates through a respective coaxial connector 138,140 with a respective probe element 30 a having a center conductor 142surrounded by a guard 144. In order to provide adequate shielding forthe coaxial cables 134, 136, especially in the region outside of theoctagonal box 48, an electrically-conductive shield tube 146 is providedaround the cables 134, 136 and electrically connected through the shieldbox 126 with the outer connector element 128 c, 130 c of the respectivetriaxial connectors 128, 130. The shield tube 146 passes through thesame slit in the foam 50 as does the underlying extending member 26 ofthe probe positioner 24. Thus, each individually movable probe 30 hasnot only its own separate individually movable probe holder 28 but alsoits own individually movable shield 146 for its guarded coaxial cables,which shield is movable in unison with the probe holder independently ofthe movement of any other probe holder by any other positioningmechanism 24. This feature is particularly advantageous because suchindividually movable probes are normally not equipped for both shieldedand guarded connections, which deficiency is solved by the describedstructure. Accordingly, the probes 30 are capable of being used with thesame guarding and Kelvin connection techniques in a ready-to-use manneras is the chuck assembly 20, consistently with full shielding despitethe individual positioning capability of each probe 30.

Preferred Alternative Embodiment of the Probe Station

FIG. 10 depicts a preferred alternative embodiment 220 of the waferprobe station which, like the basic embodiment depicted in FIG. 3, hasthe capability for providing guarded and Kelvin connections to thedevice under test but which also has additional features forfacilitating extremely sensitive low-level current measurements. Inparticular, the alternative embodiment 220 includes a fully guardedmovable chuck assembly 221 and a fully guarded probe-holding assembly223. These features are described below in further detail each under aseparate subheading.

In the respective drawings of the alternative probe station 220 and thebasic probe station, like reference numerals have been used to identifyelements that are common to both systems. Thus, comparing FIGS. 3 and10, it will be evident that the fully guarded movable chuck assembly 221is carried on a rectangular plunger 18 for movement along X, Y and Zaxes under the control of a motorized positioner 16. As indicated bydashed lines in FIG. 10, the movable chuck assembly 221 haspredetermined outer limits of horizontal movement 225 which, aspreviously described, are the result of interfering interaction betweenthe upstanding margins which are on the bottom sealing plates 72, 74,76, and 78.

FIG. 10 also shows a dashed line 227 signifying Z-axis or verticalmovement of the chuck assembly 221. The expansibility of resilientgasket 46 together with the limited vertical adjustability of the platen12 provide a further mechanism, in addition to that of the motorizedpositioner, for shifting the chuck assembly 221 vertically relative tothe upper half 42 of the environment control enclosure box. For the sakeof convenience, the upper and lower halves 42 and 44 of the controlenclosure will hereafter be collectively referred to as the outershielding enclosure 229 to emphasize their importance in providingshielding for the chuck assembly against outside electromagneticinterference. At the same time, however, it will be recognized that theouter enclosure has several other significant functions including gascontainment, light shielding and temperature control.

In certain respects, the connector mechanism 231 of the alternativeprobe station 220 resembles that of the basic probe station. Forexample, in order to enable low-voltage measurements to be made inrelation to the chuck assembly 221, the connector mechanism 231 includesboth a source line and a measurement line to provide Kelvin-typeconnections to the chuck assembly. In particular, referring also to FIG.12, the source and measurement lines each include an exterior connector232 and 233, a flexible connector assembly 235 and 237, and an interiorconnector 239 or 241, respectively. For purposes of low-level currentmeasurement, either of these lines can be used, and thus the broaderterm signal line, as used hereinbelow, will be understood to refer to aline that is of either type.

In relation to the chuck assembly 221, the exterior connectors 232 and233 are mounted, as previously, on a vertical wall of the outershielding enclosure 229 where they are accessible for detachableconnection to an external signal line (e.g., 243 or 245) which isconnected, in turn, to an external test instrument (not shown). Theinterior connectors 239 and 241 are mounted adjacent the chuck assembly221. Preferably, the flexible connector assemblies 235 and 237 eachinclude an end connecting member by which such assembly is fasteneddetachably to its corresponding interior connector so that fuller accessto the sides of the chuck assembly can be obtained, as needed, in orderto facilitate replacement of particular chuck assembly elements. Eachconnector assembly 235 and 237 is flexible in order to accommodaterelative movement between the chuck assembly 221 and the outer shieldingenclosure 229.

Preferably, the exterior connectors 232 and 233, the connectorassemblies 235 and 237 and the interior connectors 239 and 241 are eachof triaxial configuration, that is, each includes a center (signal)conductor surrounded by an intermediate (guard) conductor which, inturn, is surrounded by an outer (shield) conductor. These elements,alternatively, can be of coaxial configuration if individual lineshielding is not employed. The connector mechanism 231 as it relates tothe chuck assembly 221 is further described under the subheadingimmediately below and, in particular, it is therein described how suchmechanism differs from that of the basic probe station due to its fullyguarded construction. That portion 231 a of the connector mechanismrelating to the probe-holding assembly 223 is described below under theseparate subheading pertaining thereto.

Fully Guarded Chuck Assembly and Connector Mechanism

Referring to FIG. 10, as in the basic probe station, the chuck assembly221 of the alternative probe station 220 includes a first or upper chuckassembly element 280, a second or lower chuck-assembly element 281 and athird chuck assembly element 283 which detachably mounts the first twoelements. Referring also to FIG. 11, as in the basic system, therespective chuck assembly elements are electrically isolated from eachother including by dielectric spacers 85 and O-rings 88, and the firstchuck assembly element has an upper surface 285 for horizontallysupporting a test device, a lower surface 287 opposite the upper surfaceand a peripheral surface 289 vertically interconnecting the upper andlower surfaces.

However, in the alternative probe station 220, the construction of thesecond chuck assembly element 281 is different than that previouslydescribed in certain important respects. In particular, in addition tohaving a lower component 291, the second chuck assembly element furtherincludes a skirting component 293 and an upper component 295. Thesecomponents, as explained in greater detail below, are electricallyconnected with each other and are arranged relative to each other so asto surround the first chuck assembly element 280 on all sides. Morespecifically, a surface portion 291 a included on the lower componentextends opposite the entire portion of the lower surface 287 on thefirst chuck assembly element, a surface portion 293 a included on theskirting component extends opposite the entire portion of the peripheralsurface 289 on the first chuck assembly element and a surface portion295 a included on the upper component extends opposite the entireportion of the upper surface 285 on the first chuck assembly element.Moreover, these relationships are maintained even when the chuckassembly 221 is brought to its predetermined outer limits of horizontalmovement 225. Thus, the surface portion 295 a on the upper component ismaintained opposite the entire portion of the upper surface 285 on thefirst chuck assembly element despite relative movement occurringtherebetween.

Viewing this arrangement somewhat differently, it will be recognizedthat relative to any location on the respective surfaces 285, 287 and289 of the first chuck assembly element 280, the second chuck assemblyelement 281 is considerably closer to such location than is the outershielding enclosure 229 even along those angles of approach which do notlie perpendicular to such surfaces. Accordingly, electromagneticinteraction between the first chuck assembly element and its neighboringenvironment is only able to occur in relation to the second chuckassembly element. However, as fully described below, the connectormechanism 231 is so constructed as to enable the voltage potential onthe second chuck assembly element to follow the potential which is onthe first chuck assembly element. In accordance with this relationship,then, the first chuck assembly element is effectively isolatedelectrically from its neighboring environment.

In the preferred alternative probe station 220 depicted in FIGS. 10-12,the skirting component 293 is formed from a closed-sided strip ofconductive material such as tin-plated steel. The strip is connectedboth mechanically and electrically to the lower component 291 by aplurality of threaded steel bolts 297 petal washers 299 which are seatedon the bolts maintain the skirting component 293 in radiallyspaced-apart surrounding relationship to the first chuck assemblyelement 280. In this manner, the surface portion 293 a of the skirtingcomponent and the peripheral surface 289 of the first chuck assemblyelement are separated from each other by an open gap 301 so that thecapacitance between these respective surfaces is minimized.

Referring to FIG. 10, the upper component 295 of the preferredalternative probe station 220 is formed from a sheet of conductivematerial such as tin-plated steel. The upper side of the sheet isattached to the top of the outer shielding enclosure 229 by severalstrips of insulative foam tape having double-sided adhesive as of a typesold commercially, for example, by the 3M Company based in St. Paul,Minn. In this manner, the upper component 295 is held in spacedrelationship above the skirting component 293 so that each is separatedfrom the other by an open gap.

The above form of construction is preferred over one in which no gap isprovided between the skirting component 293 and the upper component 295as may be achieved, for example, by fitting a resilient conductivegasket to the skirting component in such a manner that the gasketbridges the gap between the respective components. In this alternativebut less desired form of construction, it is difficult to completelyavoid abrasion of the upper component because the gasket or otherbridging element will rub across the upper component when that componentshifts horizontally relative to the outer shielding enclosure 227. Inthis alternative construction, then, it is possible for small filings orother debris to be swept from the abraded surface of the upper component295 into the central testing area causing possible damage to the deviceunder test. In the preferred form of construction, on the other hand,the possibility of such damage has been avoided.

Centrally formed in the conductive sheet comprising the upper component295 is a probing aperture 307. As indicated in FIG. 10, the extreme endof each individual probe 30 can be inserted through this probingaperture in order to make contact with a wafer supported for test on thefirst chuck assembly element 280. Referring also to FIG. 14, which showsthe view looking toward the surface portion 295 a of the uppercomponent, the probing aperture 307 has an irregular diameter, that is,it is of a cross-like shape. As an option, a conductive panel 309 ispreferably provided that selectively fits detachably over the probingaperture and that includes a central opening 311, smaller in size thanthe probing aperture 307, through which the extreme end of theelectrical probe can be inserted, as shown. Because of its relativelysmaller opening, the conductive panel 309 tends to reduce somewhat therange of horizontal movement of each electrical probe but,correspondingly, tends to increase the degree of electromagneticisolation between the first chuck assembly element 280 and the outershielding enclosure 229 since it extends the effective surface area ofthe surface portion 295 a of the upper component. Hence, the conductivepanel is particularly suited for use in those applications in whichextremely sensitive current measurements are needed. Referring again toFIG. 14, the exemplary conductive panel 309 has a cross-like shape sothat it covers the probing aperture 307 with only a small margin ofoverlap. Referring to FIGS. 10 and 14 together, conductive pegs 313project outwardly from the underside of the conductive panel. Thesepegs, as shown, are arranged into opposing pairs so that each pair canbe wedged snugly between opposite corners of the probing aperture, thuspreventing rotation of the conductive panel in its seated position onthe upper component.

Referring to FIG. 13, the outer shielding enclosure 229 includes aloading aperture 315 through which access to the chuck assembly 221 isobtained and a hinged door 68 for opening and closing the loadingaperture. Along this portion of the outer shielding enclosure, the uppercomponent 295 is divided into respective first and second sections 317and 319. The first section 317 is mounted inside the door for movementwith the door as the door is being opened, and the second section 319 ismounted behind the surrounding portion 321 of the outer shieldingenclosure. As previously described, insulated foam tape havingdouble-sided adhesive is used to mount these sections so that each iselectrically isolated from its respective mounting surface. As shown inFIG. 13, the, outer edge 317 a of the first section is slightly offsetinwardly from the edge of the door 68 so that when the door is moved toits closed position in slight marginal overlap with the surroundingportion 321, this brings the two sections 317 and 319 into physicalcontact with each other along an extended portion of their respectiveouter edges. To further ensure that there is good electrical contactbetween the first and second sections of the upper component, aconductive tab 323 is soldered to the underside or surface portion 295 aof the first section so that when the door is closed such tab canestablish oxide-removing wiping electrical contact with the underside orsurface portion 295 a of the second section.

In the preferred probe station 220, not only is the chuck assembly 221fully guarded but so too is the connector mechanism 231. In particular,referring to FIGS. 10 and 12, the signal lines of the connectormechanism 231 by which the chuck assembly is energized are fully guardedby a first box-like inner guard enclosure 325 and a second box-likeinner guard enclosure 327. As is explained under the next subheadingbelow, there is also a third box-like inner guard enclosure 329 (referto FIG. 15) to provide guarding for that portion 231 a of the connectormechanism associated with each probe-holding assembly 223.

With respect to the ground connections established via the connectormechanism 231, the outer conductor of each exterior connector 232 and233 is electrically connected through the outer shell of such connectorto the outer shielding enclosure 229. Respective grounding straps 235 cand 237 c electrically interconnect the outer conductor of eachconnector assembly 235 and 237, respectively, to the outer shieldingenclosure. The outer conductor of each interior connector 239 and 241 isconnected electrically through the outer shell of such connector to thethird chuck assembly element 283 via a metal flange 331 that projectsoutwardly from the side of the third chuck assembly element.Accordingly, if detachable connection is made between either connectorassembly 235 or 237 and the corresponding interior connector 239 or 241,the third chuck assembly element 283 and the outer shielding enclosure229 are then tied to the same potential, that is, to the groundpotential of the system as maintained at either exterior connector 232or 233 via the outer conductor of the external signal line (e.g., 243 or245).

The inner and intermediate conductors of the interior connector 239 areseparated out from their respective insulating members so as to form asignal (source) line element and a guard line element 239 a and 239 b,respectively. In relation to an inner or intermediate conductor, theterm “line element” as used herein and in the claims is intended torefer to such conductor along any portion thereof where it is arrangedexterior of its outside conductor(s), even if at some portion furtherback from its end the inner or intermediate conductor is surrounded bythe outside conductor(s).

Referring also to FIG. 11, in similar manner, the inner and intermediateconductors of the interior connector 241 are separated out from theirrespective insulating members so as to form a signal (measurement) lineelement and a guard line element 241 a and 241 b, respectively. Therespective signal line elements 239 a and 241 a are electrically tiedtogether at the first chuck assembly element 280 thereby establishing aKelvin connection with respect thereto. In particular, these signal lineelements are inserted into respective holes 333 and 335 which are formedin the peripheral edge of the first chuck assembly element 280 wherethey are held detachably in place each by a respective set screw 337 or339 that is adjusted by means of turning to its respective clampingposition.

In order to provide full guarding in relation to each of the respectivesignal line elements 239 a and 241 a, a first box-like inner guardenclosure 325 is provided which is so arranged that it surrounds theseelements in interposed relationship between them and the outer shieldingenclosure 229. In the preferred embodiment depicted, tin-plated steelpanels are used to construct the first guard enclosure. In order toenable the leakage current flowing from either of the signal lineelements 239 a or 241 a to be reduced to a negligible level, each of theguard line elements 239 b and 241 b is electrically connected, as bysoldering, to the enclosure 325, preferably on an inside wall thereof.Accordingly, by appropriate adjustment of the guard potential as carriedby either guard line element 239 b or 241 b, the potential on the guardenclosure can be controlled so as to substantially follow the signalpotential which is carried either by the signal (source) line element239 a or by the signal (measurement) line element 241 a. Since leakagecurrent from either signal line element 239 a or 241 a can thus bereduced to virtually zero, the measurement of very low-level currentscan be made via either element. Moreover, to the extent that fielddisturbances occur in the region surrounding the first guard enclosure,such disturbances will be resolved at the first guard enclosure withoutaffecting the stability at the signal as carried by either signal lineelement.

As indicated in FIGS. 11 and 12, the first guard enclosure 325 has astep 341 in its floor panel so that no part of the enclosure comes intoeither physical or electrical contact with the third chuck assemblyelement 83. The first guard enclosure is electrically connected at itsinside edges 345 to the skirting component 293, as by soldering. Hencethe guard potential as carried by either of the guard line elements 239b or 241 b is conveyed to the lower and skirting components of thesecond chuck assembly element 281 via the first guard enclosure 325,thereby enabling these components to provide guarding in relation to thefirst chuck assembly element 280. The enclosure further forms a passage347 that opens towards the first chuck assembly element 280. In thismanner, the respective signal line elements 239 a and 241 a arecompletely enclosed for full guarding by the first guard enclosure 325as they extend through this passage for parallel electrical connectionwith the first chuck assembly element.

As previously mentioned, the various components of the second chuckassembly element 281 are electrically connected to each other, that is,the upper component 295 is electrically connected to the skirtingcomponent 293 as well as to the lower component 291. In order to obtainthis connection to the upper component, a coupling assembly 349 isprovided. This coupling assembly is so constructed that the guardpotential as carried by the intermediate (guard) conductor of eitherexterior connector 232 or 233 can be conveyed to the upper component viasuch coupling assembly in addition, for example, to being conveyed tothe lower and skirting components via either of the guard line elements239 b or 241 b.

Referring to FIG. 10, the coupling assembly 379 preferably acquires theguard potential at a fixed connection point located adjacent theexterior connectors 232 and 233. In preparation for this connection, theinner and intermediate conductors of the exterior connector 232 areseparated out from their respective insulating members so as to form asignal (source) line element and a guard line element 232 a and 232 b,respectively. Similarly, the inner and intermediate conductors of theexterior connector 233 are separated out so as to form a signal(measurement) line element and a guard line element 233 a and 233 b,respectively. Opposite the exterior connector 232, the inner andintermediate conductors of the connector assembly 235 are separated outto form a signal (source) line element and a guard line element 235 aand 235 b, respectively, while opposite the exterior connector 233 theinner and intermediate conductor of the connector assembly 237 areseparated out to form a signal (measurement) line element and a guardline element 237 a and 237 b, respectively. As shown, the correspondingpairs of signal line elements are directly connected electrically by,for example, soldering signal line element 232 a to 235 a (to join thesource line) and signal line element 233 a to 237 a (to join themeasurement line).

In order to provide full guarding in relation to each of thecorresponding pairs of signal line elements 232 a and 235 a or 233 a and237 a, a second box-like inner guard enclosure 327 is provided which isso arranged that it surrounds these elements in interposed relationshipbetween them and the outer shielding enclosure 229. In the preferredembodiment depicted, tin-plated steel panels are used to construct thesecond guard enclosure. In order to enable the leakage current flowingfrom either of these pairs of signal line elements to be reduced to anegligible level, each of the guard line elements 232 b, 233 b, 235 band 237 b is electrically connected, as by soldering, to the secondguard enclosure 327, preferably on an inside wall thereof. Hence, byappropriate adjustment of the guard potential as carried by either guardline element 232 b or 233 b, the potential on the guard enclosure can becontrolled so as to substantially follow the signal potential that iscarried either by the pair of signal line elements 232 a and 235 a or bythe pair of signal line elements 233 a and 237 a. Since leakage currentfrom either of the corresponding pairs of signal line elements 232 a and235 a or 233 a and 237 a can thus be reduced to virtually zero, themeasurement of very low-level currents can be made via either pair.Moreover, any field disturbances in the region surrounding the secondguard enclosure will be resolved at such enclosure without affecting thestability of the signal as carried by either pair.

Referring to FIGS. 10 and 12 together, the coupling assembly 349includes a lower guard line element 351, a pair of pass-throughconnectors 352 and 353, a flexible connector assembly or cable 355, andan upper guard line element 356. To enable the coupling assembly toacquire the guard potential, one end of the lower guard line element 351is electrically connected to the second guard enclosure 327, as bysoldering. Preferably, the pass-through connectors and the connectorassembly are of coaxial configuration so that the center conductor ofeach is able to convey the guard potential from the lower guard lineelement to the upper guard line element. The upper guard line element356 and the upper component 295, in turn, are connected togetherelectrically, as by soldering, so that the guard potential is conveyedto the upper component via the upper guard line element.

In an alternative construction, it is possible to run the lower guardline element 351 directly between the second guard enclosure 327 and theupper component 295. However, such a construction would make itdifficult to separate the upper and lower halves 42 and 44 of the outershielding enclosure 229 should the operator wish to gain access toelements within the enclosure. In order to provide such access, in thepreferred coupling assembly 349 shown, the connector assembly 355 hasend connecting members 355 a and 355 b that connect detachably to eachpass-through connector. Thus, upon detachment of either end connectingmember, the two halves 42 and 44 of the outer shielding enclosure can beseparated from each other to gain access to the interior of theenclosure.

In accordance with a preferred method of using the fully guarded chuckassembly 221, test equipment suitable for guarded measurement oflow-level currents is connected with a selected one of the exteriorconnectors 232 or 233 via an external line (e.g., 243 or 245). The firstchuck assembly element 280 is then energized, that is, a current signalis established through a signal path which includes the probe 30, thedevice-under-test (not shown), and that series of signal line elements232 a, 235 a and 239 a, or 233 a, 237 a and 241 a which corresponds tothe chosen connector 232 or 233. A nonzero signal potential is thusdeveloped on the first chuck assembly element 280 in relation to systemground, that is, in relation to the potential on the outer shieldingenclosure 229. As this occurs, a guard potential substantially equal tothe signal potential is simultaneously conveyed to the upper component295 via guard line elements 351 and 356 and to the lower and skirtingcomponents 291 and 293 via that series of guard line elements 232 b, 235b and 239 b or 233 b, 237 b and 241 b which corresponds to the chosenconnector. This guard potential is initially generated inside the testequipment by a feedback network of a design known to those of ordinaryskill in the art. In accordance, then, with the fore-going procedure,the first chuck assembly element 280 is electrically guarded by thesecond chuck assembly element 281.

Since, in accordance with the above method, almost no potentialdifference is developed between the first chuck assembly element 280 andthe neighboring second chuck assembly element 281, and since thegeometry of the second chuck assembly element is such that it fullysurrounds the first chuck assembly element, leakage current from thefirst chuck assembly element is reduced to negligible levels. A furtherreduction in leakage current is achieved by the first and second innerguard enclosures 325 and 327 which, being held at nearly the samepotential as the signal line elements they respectively surround, reduceleakage currents from those elements. As a result, system sensitivity tolow-level current is increased because the level of current that isallowed to escape detection by being diverted from the signal path isnegligible.

In addition to increased current sensitivity, another major benefit ofthe fully guarded chuck assembly 221 is its capability for reducingsettling time during low-level current measurements. During suchmeasurements, the rate of charge transfer in relation to the first chuckassembly element 280 is limited by the amount of current that can flowthrough the device under test given the bias conditions imposed on thatdevice, whereas the rate of charge transfer in relation to the secondchuck assembly element 281 is under no such restriction. Accordingly,the second chuck assembly element 281 and also the first and secondguard enclosures 325 and 327 are able to transfer sufficient charge sothat each achieves its full potential relatively quickly, even thougheach is capacitively coupled to surrounding conductive surfaces ofrelatively large area such as those on the interior of the outershielding enclosure 229. Finally, in relation to the first chuckassembly element 280 and also to the signal line elements in theconnector mechanism 231, the second chuck assembly element 281 and eachof the guard enclosures 325 and 327 act as barriers against strayelectromagnetic radiation, thereby increasing signal stability.

The benefits provided by the fully guarded chuck assembly 221 in regardto low-level current measurements are achieved while, at the same time,preserving the capacity of the system for making low-level voltagemeasurements. As previously described, the connector mechanism 231continues to provide separate source and measurement lines suitable forthe establishment of Kelvin-type connections. Moreover, the first chuckassembly element 280 is movable relatively freely relative to eachindividual probe 30 without being encumbered by any of the elements thatprovide guarding. In particular, electrical connection is maintainedbetween the upper component 295 and the skirting component 293 via thecoupling assembly 349 despite horizontal or vertical movement occurringbetween these components. With respect to the first inner guardenclosure 325 and the second inner guard enclosure 327, either verticalor horizontal movement is accommodated between these enclosures becauseof flexibility in the connector assemblies 239 and 241.

Probe-Holding Assembly With Fully Guarded Connector Mechanism

The alternative probe station 220 preferably includes at least one fullyguarded probe-holding assembly 223. Referring to FIGS. 15 and 16, itwill be recognized that from the standpoint of overall construction,each fully guarded probe-holding assembly 223 is generally similar tothe probe-holding assembly of the basic probe station as depicted inFIGS. 8-9. As between FIGS. 15-16 and FIGS. 8-9, like reference numeralshave been used to identify elements common to both systems. It will beseen, in particular, that the portion 231 a of the connector mechanismassociated with the probe-holding assembly 223 preferably includes apair of connectors 128 and 130 of triaxial configuration, each of whichare mounted on an outer shielding enclosure or box 126. These exteriorconnectors, then, are suitably configured to receive the respectivesource and measurement line cables 132 which arrive from the externaltest instrument (not shown) as needed to establish Kelvin-typeconnections in relation to the probe 30.

The inner and intermediate conductors of the exterior connector 128 areseparated out from their respective insulating members so as to form asignal (source) line element and a guard line element 128 a and 128 b,respectively. Similarly, the inner and intermediate conductors of theexterior connector 130 are separated out from their respectiveinsulating members so as to form a signal (measurement) line element anda guard line element 130 a and 130 b, respectively. As in the basicsystem shown in FIGS. 8 and 9, each of the signal line elements 128 aand 130 a is electrically connected with the center conductor 142 of arespective probe element 30 a via the center conductor of acorresponding coaxial connector 138 or 140 and the center conductor of acorresponding coaxial cable 134 or 136. To provide a guarding capabilityin relation to each signal path, each guard line element 128 b or 130 bis electrically connected with the guard conductor 144 of itscorresponding probe element 30 a via the outside conductor of thecorresponding coaxial connector 138 or 140 and the outside conductor ofthe corresponding coaxial cable 134 or 136. Each exterior connector 128or 130 further includes an outer shield element 128 c or 130 c both ofwhich are electrically connected with the outer shielding box 126. Thisbox, in turn, is electrically connected with the shield tube 146, sothat when the shield tube is inserted into the octagonal steel box 48,as previously described, the signal and guard lines will be fullyshielded.

In order to provide full guarding in relation to each of the respectivesignal line elements 128 a and 130 a of the fully guarded probe-holdingassembly 223, the alternative probe station 220 includes a thirdbox-like inner guard enclosure 329. This guard enclosure is so arrangedthat it surrounds the respective signal line elements 128 a and 130 a ininterposed relationship between them and the outer shielding enclosureor box 126. In the preferred embodiment depicted in FIGS. 15 and 16, thethird guard enclosure is constructed from tin-plated steel panels. Therespective guard line elements 128 b and 130 b are both electricallyconnected, as by a respective wire 148, to the enclosure 329, preferablyon an inside wall thereof.

During the measurement of low-level currents through the probe 30, aspreviously described, the interconnections made between the connectormechanism portion 231 a and the third guard enclosure 329 enable thepotential on the guard enclosure 329 to be controlled so that suchpotential substantially follows the signal potential as carried byeither signal line element 128 a or 130 a. In particular, the potentialon the third guard enclosure is controlled either by adjustment of theguard potential on guard line element 128 b or 130 b.

Since, in accordance with the above construction, the third guardenclosure 329 fully surrounds each signal line element 128 a or 130 aand will carry substantially the same potential as these elements,leakage current from either signal line element is reduced to virtuallyzero so that very low-level currents can be measured via either element.Moreover, any field disturbances in the region surrounding the thirdguard enclosure will be resolved at that enclosure without affecting thestability of the signal as carried by either signal line element.

Although a preferred alternative embodiment 220 of the probe station hasbeen described, it will be recognized that alternative forms of theembodiment are possible within the broader principles of the presentinvention. Thus, with respect to the fully guarded chuck assembly 221,instead of having a closed-sided structure, either the skirtingcomponent 293 or the upper component 295 may have a mesh, open-slat ormultilevel structure. Also, it is possible to position a dielectricsheet between the first chuck assembly element 280 and the skirtingcomponent 293 in order to form a sandwich-type structure. In yet afurther possible modification, the first inner guard enclosure 325 canbe integrated with the skirting component 293 so that, for example, theskirting component includes U-shaped side portions which serve as thefirst guard enclosure. Moreover, instead of having a box-like form, eachguard enclosure can take the form of a cylinder or various other shapes.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A probe station having a chuck for supporting a semiconductor undertest comprising: (a) a screw with a screw head and a shaft supported bysaid chuck; (b) a conductive member laterally surrounding and spacedapart from said chuck, wherein said shaft supports said conductivemember and said screw head is exterior to said conductive member; (c) aspacer surrounding said shaft and positioned between said conductivemember and said chuck, wherein said spacer and said conductive memberare at the same potential; and (d) a probe for testing a semiconductorresting on said chuck.
 2. The probe station of claim 1 wherein saidconductive member comprises a skirt extending vertically above saidchuck.
 3. The probe station of claim 1 wherein said conductive member isa sheet-like shield.
 4. The probe station of claim 1 wherein said screwpasses through an oversized aperture in said chuck thereby preventingcontact between said shaft and said chuck.
 5. The probe station of claim1 wherein said spacer is constructed of a material having low dielectricconstant.